1. Field of the Invention
The present invention is directed to an OCT (optical coherence tomography) method and apparatus, and in particular to such a method and apparatus for obtaining a sequence of OCT images and for subsequently combining those images into a diagnostic image.
2. Description of the Prior Art
Optical coherence tomography (OCT) is an invasive imaging modality that is currently used for visual assessment of lesions or pathologies in blood vessels, such as stenoses and problematic plaque deposits. To improve such assessment, it is desirable to produce a diagnostic image wherein the portion of the vessel in question is displayed so that the pathology can be intuitively recognized without additional technical outlay, and without additional radiation exposure to the patient.
Arteriosclerotic pathologies of the coronary vessels are the primary cause of death in industrialized nations. Narrowing of the coronary vessels (stenoses) or lipid-filled plaque deposits are the most frequent cause of heart attacks. Prevalent therapy measures are balloon dilation and/or the implantation of stents. A significant pre-condition for the therapy selection and the therapy success is a precise characterization of the pathological situation (lesion).
In addition to determining the length (extent in the vessel) of the lesion, the degree of constriction of the vessel, the diameter of the original, healthy vessel and the wall structure of the vessel are the types of information which contribute toward such a precise characterization of the lesion. It is desirable to know such information not only in the region of the lesion itself, but also at a region or regions downstream from the lesion in terms of blood flow, and a region or region upstream from the lesion in terms of blood flow.
OCT is a known imaging modality that allows images to be obtained from the inside of a vessel using an intravascular imaging catheter. A general description of OCT is available, for example, from PCT Application WO 97/32182. OCT systems operate in a light wavelength range of approximately 1300 nm (near-infrared range). Light in this wavelength range is emitted from a lens of the catheter into the vessel wall, and the reflection of the emitted light from the vessel wall is detected with depth resolution by interferometry. Image information is obtained from various adjacent points of the vessel wall by rotation of the radiated light beam, and this image information is combined into a 2D image representing a “slice” of the vessel in the plane of the radiated light beam. Additionally, the catheter can be moved along the longitudinal direction of the vessel during image acquisition, in order to obtain successive images of the vessel. The OCT catheter is inserted into the vessel up to a selected point, and is then withdrawn in a continuous, monitored movement, known as a “pullback,” during which images are successively obtained, so that a “stack” of two-dimensional slice images is acquired. These images can be combined offline (i.e., after acquisition) to form a three-dimensional dataset. In general, however, only the current two-dimensional slice image is actually visible on the display screen that is used during the image acquisition. These individual OCT images have a very high spatial resolution. Dependent on the distance of the vessel wall from the catheter, the resolution is below approximately 40 μm in the x, y direction (slice plane) and is in the range between 40 to 100 μm in the z-direction (pullback direction), depending on the frame rate and withdrawal speed. For example, the light beam can be rotated in the slice plane at a rotational frequency of up to 30 Hz.
For characterization of a stenosis in the catheter laboratory (cathlab) a series of techniques are currently available, but each has certain inadequacies associated therewith.
The most important conventional method for characterization of a lesion is angiography. Angiography equipment available today provided information only about the vessel itself, but does not supply information regarding the morphology (appearance and extent) of the lesion. Nevertheless, angiography results can be used for further therapy selection. In addition to the initial assessment of vessel constrictions that can be seen in the angiography image, automated evaluation programs can be used for quantitative coronary angiography (QCA). An example is the Quantcor software available from Siemens AG. Such programs typically provide information about the length of a stenosis, the reference diameter (vessel diameter before and after the lesion, minimum/average/maximum vessel diameter, and maximum diameter reduction in the constriction).
The lack of information regarding the vessel structure in the region of the lesion, and the two-dimensional nature of angiography exposures are limiting factors in assessing a therapy using this technique. Errors primarily occur in the length measurement, due to the lesion not lying precisely in the plane of the angiography image, and thus appearing foreshortened in the angiography image. This type of problem, however, can be addressed by the use of three-dimensional reconstructions from two x-ray projections using software also available from Siemens AG known as AXIOM ARTIS with Interventional Cardiac 3D (IC3D). This requires, however, that the patient receive two x-ray doses. Intravascular imaging using IVUS (intravascular ultrasound) can provide additional information regarding the structure of the vessel wall. Background concerning intravascular ultrasound can be found in “Intravascular Ultrasound: Novel Pathophysiological Insights and Current Clinical Applications,” Nissen et al., Circulation (2001) pages 604-616. All significant, therapy-determining quantities of the lesion can be determined with this two-dimensional slice imaging technique. IVUS therefore has become the most prevalent technique for characterization of stenoses and plaque deposits. Like OCT, IVUS involves the withdrawal (pullback) of a probe, in this case an ultrasound probe, through a vessel. Despite the aforementioned advantages of IVUS, the evaluation of IVUS pullback sequences is subject to limitations.
Due to movement of the heart through successive cardiac cycles, a shifting of the catheter in the vessel ensues in the imaging plane, as well as periodically along the vessel axis (longitudinal direction). This problem is described in “Axial Movement of the intravascular Ultrasound Probe During the Cardiac Cycle: Implications for Three-Dimensional Reconstruction and Measurements of Coronary Dimensions,” Arbab-Zadeh et al., Am. Heart J., Vol. 138, No. 5 (1999), pages 865-873. Analyzing the image sequence while taking these factors into account requires experience on the part of the interpreting physician, particularly when comparing various vessel sections (longitudinal scans). Automatic quantifications of IVUS images would require an automatic segmentation of the IVUS images, which currently can be achieved only to a limited extent due to the limited resolution, the fluctuating intensities and contrasts, and by artifacts in the IVUS images.
An improved approach is known as ANGUS, and is described in the article “True 3-Dimensional Reconstruction of Coronary Arteries in Patients by Fusion of Angiography and IVUS (ANGUS) and Its Quantitative Validation,” Slager et al., Circulation (2000) pages 511-516. In this technique, pullback of the IVUS catheter is registered with a biplanar x-ray system from two projection directions. The three-dimensional course of the vessel then can be reconstructed from this information, and with ECG triggering of the images. Such a biplanar x-ray system, however, is available only in a limited number of catheter laboratories, and also requires an additional radiation exposure for the patient.
An alternative to x-ray imaging with ECG triggering is the use of a positioning system, wherein a sensor is mounted at the tip of the imaging sensor, the sensor providing an indication of the position and orientation of the imaging probe to an extracorporeal positioning system. Such a technique is described in U.S. Pat. No. 5,830,145. The use of the additional extracorporeal positioning system and special catheters, however, are necessary to implement this technique.
A method is described in “Four-Dimensional Coronary Morphology and Computational Hemodynamics,” Wahle et al., “Medical Imaging 2001: Image Processing” Sonka et al. eds. (pages 743-753) wherein the ECG is recorded in parallel with the IVUS (ANGUS) pullback, and thus a cardiac phase, determined or selected from the ECG can be associated with the individual IVUS images. A 4D model of the vessel section, used to simulate blood flow in the vessel, can be determined from this information.
It is also possible to visualize the consistency (composition, formation) of the vessel wall using OCT. The determination of all significant quantities is additionally possible for characterization of a lesion. An advantage of OCT is the high resolution of approximately 40 μm, a good high-contrast representation of the vessel sections, and the absence of shadowing effects due to calcifications.